Turkish prime minister Ahmet Davutoğlu, minister of transport Feridun Bilgin and former minister of transport Lütfi Elvan gathered in Istanbul to watch the breakthrough on the Eurasia tunnel last week.

The event marked a milestone in what has been described as “one of the world’s biggest and most challenging tunnel drives”.

The 13.66m diameter, 5.4km long Eurasia tunnel will carry road traffic - on two deck levels - below the Bosphorus cutting journey times from 100 minutes to around 15 minutes. However, joint venture contractors Turkey’s Yapı Merkezi Construction and Korea-based SK Engineering and Construction had to deal with fractured rock and soft ground with water pressures of up to 11bar to achieve last week’s breakthrough.

The tunnel was driven using a Herrenknecht tunnel boring machine (TBM) that was designed specifically for the project. The TBM cutterhead was built with both hard rock discs cutters, mounted on six radial arms, scrapers and buckets and openings between for the slurry support and spoil removal. A new monitoring system was used on the TBM for the first time to check the performance of the disc cutter system, measuring key values like rotation and temperature, to ensure breakdown was avoided.

With water depths of up to 62m above the tunnel alignment and soft alluvial material extending another 40m below that, the tunnel was driven at a depth of 106m below sea level. The tunnel will is part of a new 14km highway system.

In addition to dealing with the ground conditions, the tunnel also had to be designed to cope with seismic events. Special yielding joint technology has been applied at two critical points, the joint segments produced and tested in Japan.

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